In general, calcium carbonates are mainly classified into “natural calcium carbonate” prepared from natural limestone, weathered shell or the like by mechanically grinding and classifying it and “synthetic calcium carbonate” (precipitated calcium carbonate) prepared from limestone by chemically reacting it. Known processes for synthesizing the synthetic calcium carbonate include the carbonation process, the lime-soda process, and the Solvay process, among winch the lime-soda process and the Solvay process are sometimes employed for special applications while industrial synthesis of calcium carbonate typically involves the carbonation process.
Synthesis of calcium carbonate by the carbonation process involves reacting quick lime and carbonic acid gas, and typically comprises a slaking step in which water is added to quick lime CaO to give slaked lime Ca (OH)2, and a carbonation step in which carbonic acid gas CO2 is injected, into the slaked lime to give calcium carbonate CaCO3. At present, various techniques for controlling the particle shape or particle size or the like of the product calcium carbonate have been proposed by regulating reaction conditions in synthesis steps of calcium carbonate, particularly the carbonation step.
For example, patent documents 1 and 2 describe controlling the morphology or the like of calcium carbonate by adding a chelating agent during the carbonation step. Specifically, patent document 1 proposes a process for preparing well-dispersed calcium carbonate with little secondary aggregation by adding a material capable of forming a complex with metal ions during the carbonation reaction. On the other hand, patent document 2 proposes a process for preparing calcium carbonate having uniform mesopores by adding a sequestering agent at multiple stages during the carbonation step. Further, patent document 3 proposes controlling the shape of calcium carbonate by performing the carbonation reaction at two stages under specific conditions.
In addition, patent document 4 describes a technique for preparing calcium carbonate by supplying a reaction vessel with a suspension containing lime screen residues and a gas containing carbon dioxide while mixing them by an injector.